The research objective of this Faculty Early Career Development (CAREER) Program project involves cross-disciplinary applications of instrumented indentation to the investigation of elastic-plastic deformation in a number of inorganic and biological materials systems of increasing visibility and fundamental importance to the engineering and physiological communities, and spanning nano- to macro-scopic size scales. The power of indentation as a scientific tool lies primarily in its experimental simplicity, due to the minimal specimen preparation involved. However, interpretation of results is non-trivial and key to successful analysis are sufficient analytical modeling and supplementary observation. In this plan, indentation will be used to develop constitutive behavior of three 'testbed' systems: i) rapidly quenched small-volume structures on substrates, ii) carbon nanotube arrays on substrates, and iii) lung parenchyma.
Academic, industrial and/or clinical collaborators have been identified to add perspective and disseminate information. Efforts are planned to develop new interdisciplinary research courses for university, and practical and pre-collegiate training programs for K-12 students.
In this program we explored the usage of indentation as an enabling tool to pose questions about mechanical and functional behavior of clinically and technologically important systems. The activities in this program are linked, and have spurred several intellectual growth areas, across sizes as disciplines. A few are highlighted here. A. Respiratory physiology and mechanics of lung – One thrust of my NSF CAREER Award was to use macro-indentation to induce and study atelectasis in lung, and develop a set of criteria for its occurrence. This activity has grown significantly since its origin: load-controlled indents on lung showing a significant dependence of ‘hardness’ on inhaled gases. Since then, my group has developed instrumented indenters for lung, showing significantly increased stiffness and hysteresis in mechanical response in lung inflated with 100% oxygen. We have imaged and begun to model collapsing alveoli by developing an indenter that allows in-situ subsurface observation using optical coherence tomography (OCT). (This has led to collaborations with a fellow optics expert to develop the OCT-indenter for other tissues in future, using sub-surface strain fields as a detector of heterogeneities such as lesions.) The indentation work on lung clearly showed a short term effect of oxygen on lung mechanics, and likely pulmonary surfactant. Accordingly, my group has developed highly repeatable surface-tension measurements on isolated pulmonary surfactant, that have recently shown for the first time a significant alteration of surfactant transport with gaseous environment. (This required us to develop expertise in this technique in less than a year, to explore our hypotheses.) The highly translational implications of this work include (i) mechanisms of oxygen toxicity that maybe can be circumvented; (ii) ventilation strategies to enable safe delivery of oxygen; (iii) systematic, high-throughput studies of ventilation strategies to minimize alveolar over-stretch during re-inflation; and (iv) new formulations for surfactant-based drug delivery. Indentation has enabled substantial new intellectual opportunities for the next several years. B. Nano-indentation of thermally sprayed splats – Another thrust of my CAREER Award was to use nano-indentation to explore plasticity in impacted and rapidly solidified droplets. Several indentation tests displayed a significant drop in elastic modulus (as measured via Oliver-Pharr method) with depth, not explainable by literature studies on size effects, etc. This spurred two parallel efforts to determine the origin of such behavior. One was a continuum-based indentation study, exploring the effects of plastic anisotropy on indentation response. This has led to new developments in indentation methods to explore plastic anisotropy in larger systems, including sheet metals, films and coatings. The second effort was direct observation of the underside of metallic splats, revealing a highly nanoporous structure. Our analysis strongly suggested this was due to rapid depressurization and nucleation of gas bubbles in spreading droplets, post-impact (see publications.) This work continues in the thermal spray field, and has led to a collaboration with an Earth Science colleague at Rice University in volcanology, to study the parallels between thermal spray processing and volcanic phenomena. This is a highly-visible collaborative effort with potential benefit for novel materials fabrication, and deeper understanding of geologic processes by engineering the tools of scientific discovery. C. Plastic anisotropy - Many important materials systems such as coatings and sheet metals have different plastic properties in different directions, and these are difficult and laborious to measure. Indentation typically is not well-suited to measure anisotropy because it more or less 'averages' all directional properties, and hides such effects. However, if we indent near the edge or corner of a material, the sensitivity begins to re-appear. In this program our group has begun to develop formulae for industrial researchers to measure property anisotropy by indenting a material near its corners. This method will not only save time and cost, but can also be applied to higher temperature behavior, increasing its impact. Indentation has enabled several new investigations in my group by uncovering new questions about materials, and clinically-relevant phenomena. Moreover, we have demonstrated the ability to follow up these questions beyond contact mechanics, to create fruitful new directions for intellectual and translational studies.
